the past, present and future for selenium treatment 2008 wv amd task force meeting james j. gusek,...
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The Past, Present and Future for Selenium Treatment
2008 WV AMD Task Force MeetingJames J. Gusek, P.E.,Kevin W. Conroy, P.E.,Thomas Rutkowski, P.E.
Where is it found? Health Food Store Shelves Paint, pigments, dye formulating Electronics Glass manufacturing Insecticide production Pulp and paper Ash piles, FGD blowdown, Coal/oil combustion Agricultural water Petroleum processing Mining operations (1st noted in
ASMR paper, 1988)
What’s the problem?
Aquatic life hazard1983 – Kesterson National Wildlife
Refuge - CaliforniaBirth defects/death of birds, small
animals, fishSelenium cycle not well understood
Uncertainty on bioavailabilityEven if bio-available, what is toxic?
Often times low concentration, high volume - makes treatment expensive
How is it regulated?
5 µg/L Freshwater aquatic life 50 µg/L Primary DWS MCLU.S. Fish and Wildlife Service has
recommended 2 µg/L to protect fish, waterfowl and endangered aquatic species
Chemistry
4 primary oxidation states elemental selenium Se0
-2 selenide Se-2
+4 selenite (HSeO3- and
SeO3-)
+6 selenate (SeO4-2)
Chemical equilibrium principals don’t really apply
Driven byRedox conditionsBiological activitySorption processes
Overburden Geochemistry
Dzharkenite (FeSe2) (analogous to pyrite)
An organic diselenideSelenium-substituted pyrite
Three Selenium Mineral Phases ID’d in Western US Phosphate Resource Area
Ref: Ryser, et al., 2005
Factoid #2: Se precipitation in treatment systems is typically believed to be Se0
Factoid #1: Oxidized Se in weathered shale overburden originally started as selenium mineral analogous to pyrite
Past and Present - EPA BAT
Ferric coagulation/filtrationTypically pH <7Co-precipitation effectEffective removal requires
reduction of selenate to seleniteProblem if arsenic present
Lime softeningRever$e o$mo$i$
Non-preferential processPretreatment due to other typical
mine water issues may be required
Past and Present - EPA BAT
ElectrodialysisAlumina
Selenite adsorbed at pH range of 3 – 8
Silica can interfere at pH >4Selenate adsorption is poor
Ion exchangeNeed oxidized divalent selenateCompeting ion effects can hinder
effectivenessSome specialty resins tested
Past and Present - EPA BDAT
Ferrihydrite precipitation with concurrent adsorption of selenium on the ferrihydrite surface
For adsorption – need ferric ion (Fe+3) present
Most effective removal at pH 4-6Somewhat effective up to pH 8Phosphate, silicate, arsenic,
carbonate can interfere
Past and Present – EPA/DOE MWTP
Selenium Treatment/Removal Alternatives Demonstration
Report issued in 2001Three technologies tested in field
Ferrihydrite Adsorption (baseline)Catalyzed CementationBiological Reduction
One technology tested on bench scale
Enzymatic Reduction
Past and Present – EPA/DOE MWTP
Objective – treat to <50 µg/L SeWork done in 1999-2001Basis – Kennecott Utah Copper
Corp’s Garfield Wetlands-Kessler Springs site
<50 to >10,000 µg/L Se 95%+ selenateTDS 1,000 - 5,000 mg/L
Past and Present – EPA/DOE MWTP
FerrihydriteDid not work on a consistent basisVarious iron types, concentrations
and ratios usedCould achieve objective but at
prohibitive reagent consumptionQuestions on TCLP stability
Past and Present – EPA/DOE MWTP
Catalyzed cementationDeveloped for arsenic, selenium,
thallium removalRemoves metals by cementation
on the surface of iron particlesBelieved to work on both selenite
and selenateProprietary catalysts usedBench test work had shown
favorable resultsDid not work on a consistent basis
Past and Present – EPA/DOE MWTP
Biological Reduction (BSeR™)Used anaerobic solids bed
reactors Selenium reduced to elemental
selenium by biofilms and proprietary microorganisms
Molasses used as carbon sourceWas able to consistently meet
objectiveOver 70% of samples less than
detection (2 µg/L)
Past and Present – EPA/DOE MWTP
Economics
BDAT Cementation BSeR™Capital $1.0M $1.1M $0.6M
O&M $2.1M $1.2M $0.14M
NPV $17M $9.5M $1.1M
$/1,000 gal $13.90 $8.17 $1.32
$/kg Se $1,836 $1,079 $174Based on 300 gpm plant, 2 mg/L selenium influent
2001 dollars
2005 price for Se = $110/kg
Problems With the Past and Present
Non-selective processesLarge amounts of secondary wasteMultiple reagentsOkay for bulk selenium removal with
other metalsCan’t consistently get to <10 µg/L
Biological Reduction – GeneralStudied for decades (1994 ASMR)
Microbes degrade/transform contaminant because
Se used as energy sourceDetoxification mechanism for bugsResembles another ion (sulfate)Combination of the above
Anaerobic reactorsReduction to elemental seleniumNitrate/sulfate interference?Facilitated with iron presence?
90%+ removal reported
Biological Reduction – Ponds/Wetlands
Panoche Drainage District – San Joaquin Valley
74-1,400 µg/L due to Se rich soilPrimarily selenate formNumerous bioremediation studiesAlgal-Bacterial Selenium Removal
(ABSR)Anoxic ponds – reduce selenate to
selenite to elemental and settleGenerally about 80% maximum
removal
Biological Reduction – Ponds/Wetlands
Additional California work - 2005Constructed wetlands9 plant species tested63% - 71% removal~20 µg/L influent, 3 – 6 µg/L
effluent
Problem with ponds/wetlandsHRT’s in days
Biological Reduction – Advances
More selective microbes isolatedAdvances in fixed film/biofilm
mediaBetter understanding of operating
conditionsResult – retention times have
been reduced from days to hours for active systems
Advances also made in passive technology
Biological Reduction – Advances
ABMet®Offered by GE Water and Process
TechnologiesSame as BSeR™ processSeveral FGD projects at
commercial scale3,000 – 5,000 µg/L seleniumUp to 20,000 mg/L chloride98% – 99% removal projectedEffluent as low as 10 µg/L
If It’s Not a BLACK BOX, What Is Passive Treatment?
It’s the: Sequential Ecological eXtraction
Of metals in a man-made but naturalistic bio-system
Early Passive Treatment Data
Site/ Date
GPM pH Influent Se ug/L
Effluent Se ug/L
Percent Removal
NV Gold Tailing (Aerobic) 1994
10 7.5 40 16 60%
NV Waste Rock (BCR)1994
6 2.7 22 <5 >78%
Brewer Mine (BCR)1995
1 2 1,500 50 97%
Biological Reduction – AdvancesPassive Selenium Reducing BioreactorTested on Colorado Western SlopeBureau of Reclamation Science and Technology ProgramFIW Influent typically ~20 µg/LSpike to 70 µg/L2006-2007
1,000-2,000 mg/L SO4-2 background
Biological Reduction – 2007Passive Selenium Reducing Bioreactor Four reactors with different substrate
compositions Organic substrate composed of wood chips,
hay, manure ZVI incorporated; no advantage 12 hour detention time adequate, optimization
possible Operated for 20 weeks Effluent typically <2 µg/L Up to 98% removal 1 gpm pilot to be built June 2008 Full Scale unit costs: $0.26 per 1000 gallons or
$115 per kg Selenium Design similar to sulfate reducing bioreactor
Biological Reduction – Advances
Active Anaerobic Bioreactor SystemWaste rock seepage250 gpm capacityFixed film bioreactor with high
surface area mediaMolasses used as carbon sourcePhosphate/urea addedReverse osmosis system used
during high flow – 700 gpmBioreactor feed switched to RO brine
Biological Reduction – Advances
Active Anaerobic Bioreactor System 18 hour retention timeLow flow (raw seepage)
Se ~30 µg/LSO4
-2 ~6,000 mg/L
High flow (RO brine)Se ~70 µg/LSO4
-2 ~13,500 mg/L
Biological Reduction – Advances
Costs: $5.71 per 1,000 gallons (RO~$4.40?)$50,000 per kg Se removed (v. dilute)
Biological Reduction – Advances
Building (RO, filters) 50ft x 135ft
Bioreactors160ft x 60 ft
Overall120ft x 200 ft or
0.55 acres
Biological Reduction – Advances
Active Anaerobic Bioreactor System Effluent goal is 10 µg/LPilot plant operated for 7 monthsHigh sulfide was an issue
Discharge qualitySolids fouling
Full scale system designed and constructed
Operating for about 18 months Compliant water being produced
Future?Selenium with other metals
Conventional lime-iron based processes for bulk removal
Biological polishing processLow concentration selenium
Biological reductionBoth active and passive options
Combinations with other processes – i.e. membranes
Reduce costs to <$5.00/Kgal? Maybe down to $0.25/Kgal with passive BCRs?
Biological Reduction – Application Keys
Nutrients are vital in establishing microbial population
Understanding of site chemistry and environmental interactions (e.g., hydrogeology)
Analytical methodsCan get discrepancies in total and
dissolvedPossibly related to digestionVolatile selenide
Need for aerobic post-treatmentHigh COD, P, N
Conclusions
There is no silver bullet for selenium removal to low levels
Selenium can be removed to
<10 µg/LAll sites must be evaluated
individuallyPure “paper” designs are risky –
bench & pilot development work always recommended
Cost of selenium reduction to low levels is decreasing